The present invention generally pertains to calibration of receivers and transmitters and is particularly directed to calibration of receivers and transmitters to compensate for frequency dependent I/Q imbalance.
An I/Q imbalance occurs when there are amplitude and phase imbalances in the amplitude and/or the quadrature phase of analog in-phase (I) and quadrature (Q) signal components. Frequency dependent I and Q imbalances may be caused by frequency sensitive elements, such as filters, within circuit paths through which the analog I and Q components pass.
The amplitude error is manifested as a signal gain at the image frequency of the signal frequency. The image frequency is a frequency at which an image of a signal at a desired signal frequency also occurs.
The I/Q imbalance is worse in a wideband system that is adapted for demodulating one or more of a plurality of signal carriers at different frequencies than in a system that is adapted for demodulating a signal carrier at a single predetermined frequency, since the single-frequency system can readily filter the signal so that the image frequency is outside of the band of interest.
Calibration of a receiver to compensate for a frequency-dependent imbalance in the amplitude and/or the quadrature phase of analog in-phase I and Q signal components is described in “Digital Techniques for Wideband Receivers” by James Tsui, pp. 256-260, Artech House, 1995. Tsui states that if the imbalance can be measured, it can be corrected. Tsui then proceeds to derive equations for estimating correction coefficients by processing metrics which are produced by sampling the I and Q components four times at a sampling rate that is four times the input frequency. The metrics that are used to estimate the correction coefficients include S(0), S(1) and S(3). S(0) is a measurement of the DC offset of the particular I/Q component; S(1) is a measurement of the gain of the particular I/Q component at the input frequency; and S(3) is a measurement of the gain of the particular I/Q component at the image frequency. An I-channel correction coefficient is estimated by calculating:
  E  =      -          Re      ⁡              [                              2            ⁢                                          S                t                            ⁡                              (                3                )                                                                                        S                t                *                            ⁡                              (                1                )                                      +                                          S                t                            ⁡                              (                3                )                                                    ]            A Q-channel correction coefficient is estimated by calculating:
  P  =      -          Im      ⁡              [                              2            ⁢                                          S                t                            ⁡                              (                3                )                                                                                        S                t                *                            ⁡                              (                1                )                                      +                                          S                t                            ⁡                              (                3                )                                                    ]            where: Si*(1) is the conjugate of S(1).
Tsui states that his correction method should be tested at different frequencies and that the correction can be applied one frequency at a time. Tusi further states that his calibration method might be tedious to apply.
It is also known to calibrate a transmitter to compensate for a frequency-dependent imbalance in the amplitude and/or the quadrature phase of analog in-phase I and Q signal components that are passed through transmitter circuit paths that may cause said imbalance.